1. Field of the Invention
The present invention relates to methods and apparatus for controlling the position of a longitudinally moving belt and, more particularly, to such methods and apparatus by which the belt may be controlled with high precision. The present invention also relates to methods and apparatus for controlling lateral image registration on a longitudinally moving belt.
2. Description of the Related Art
In digital methods for providing color images by xerography, for example, each image area on a photoconductive belt must make at least four passes relative to a transverse line scan by a modulated laser beam or a linear array of light emitting diodes and registered to within a maximum of a 0.1 millimeter circle or within a tolerance of .+-.0.05 mm. Source imaging methods require specifications that could be four (4) times as stringent, i.e., a 0.025 mm circle or .+-.0.0125 mm tolerance. The use of timing marks spaced longitudinally on the belt in correspondence with the image areas, coupled with available electronic sensors and associated circuitry has enabled longitudinal registration of the image areas, or registration in the direction of belt travel, acceptably within this degree of precision. Registration of image elements or "pixels" in a transverse direction, or in a direction perpendicular to the direction of belt travel, has been more difficult to attain due to manufacturing tolerances in belt width, supporting roller geometry, uneven belt stretch or a combination of such tolerances. As a general rule of thumb, to achieve this degree of registration precision in the transverse direction, the change of lateral position of the photoconductor (PR) belt may not exceed .+-.0.25 mm over three consecutive images. Overall, the lateral belt position must be held in a range of about .+-.0.1 mm.
Although many types of steering systems have been devised for maintaining alignment of endless belts generally, recent developments in belt tracking control for copiers and printers have been inclined to "active" steering systems in which an electric motor drive tilts the steering roll in response to controller commands. With the help of signals from an opto-sensor, a micro processor controller determines once in each belt revolution whether the belt is on one side or the other of the set point position. Corrective steering commands are then deduced from the combination of the relative belt position and the number of iterations according to a specialized set of control rules. This system achieves lateral control within .+-.0.3 mm.
A diagonal line sensor has recently been applied to belt tracking control called "phase plane control," using a similar mechanism. Belt position is proportionally measured once per revolution, and the additional information thus available from the diagonal line sensor enables improved corrective action. Steering commands are deduced from the combination of position, its rate of change, and the number of iterations according to a specialized set of control rules. This system has a position measuring range of 14 mm peak-to-peak, but the displacement rate is maintained low enough to achieved an error of 0.3 mm over consecutive color separations.
U.S. Pat. No. 4,557,372 discloses an alignment apparatus for a belt system which is representative of belt control schemes that rely on independent measurements of two or more target patterns on the belt. In these types of systems, lateral displacement of each individual target is sensed and a displacement signal input to a belt steering device. The major difficulty with this type of belt alignment control is that the accuracy of alignment is dictated by the accuracy of the individual targets on the belt. In order to achieve target precision within .+-.0.1 mm, the costs of manufacturing the belt and providing the targets becomes excessive.
Current color printers use relatively thin and fragile plastic film based photoconductive belts. These belts are produced as long continuous coated strips. The finished photoconductive belt is made by slitting the strip to width, cutting to length, die-punching slits and holes as required, and joining by means of a seam to make a loop. Such manufacturing processes do not contribute to the kind of precision required of belt-carried indicia to provide belt alignment within 0.1 mm using existing belt steering technology.
U.S. Pat. No. 5,208,796 proposes a method and apparatus for transverse registration of image exposures on photoconductive belts subject to lateral deviation from linear travel in which targets, corresponding in location to the image areas to be exposed, are used for the detection of lateral belt displacement and to control the transverse location of exposure scans. The targets are of a pattern defining a reference line and a line inclined with respect to the direction of belt travel so that the duration of time between passage of the target lines with respect to a spacially fixed sensing axis will vary with lateral displacement of the belt. The targets may assume a variety of specific patterns and the teachings of that patent are applicable to single and multi-pass image exposure systems as well as to both modulated laser and light emitting diode types of raster output scanners or other exposure devices.
It has been found that freshly replaced or new continuous belts of the type described above seem to converge to a quasi-steady state whereat an average edge profile can be computed. This quasi-steady state in lateral belt edge deviation is realizable using tracking control algorithms well-known to those skilled in the art. However, actual tests reveal edge position fluctuations having peak values of about 0.5 mm, which does not meet published registration requirements for quality color printing. Major contributors to these apparently random wanderings of the belts are believed to be run out of the belt module rolls and imprecise steering roll control.
A major disadvantage of the above belt tracking control mechanisms, methods and algorithms is that they are not accurate enough to maintain the lateral belt position to within the range of about .+-.0.1 mm.
Accordingly, there is a need for methods and apparatus for lateral registration control of an endless belt which are accurate, inexpensive and suitable for use in such applications as color printing. Furthermore, such methods and apparatus should be capable of use with new or freshly replaced belts which tend to naturally migrate from a quasi-steady state at a first rate over a period of time, as well as with older belts which can migrate or otherwise randomly wander at a different second rate through continued use thereof.